Abstract Radioactivity and decay power of spent MOX fuel are very crucial in term of storage or disposal. In this work, the uncertainty of actinide decay heat introduced by the uncertainty of nuclear data and the prediction accuracy of isotopic generation was studied. 100 year after discharge, for example, a 5% error on the decay heat will be introduced by a 20% uncertainty on the Pu-240 capture cross section through Am-241 production. We performed sensitivity analyses with covariance data in JENDL-3.3 of cross sections of notable nuclides on each energy group. As an example, standard deviation of the calculated actinide decay heat 100 year after discharge is evaluated to be 1.26% on a 1σ base.